Schema upgrades were buggy from 4->5 (the v5 would be skipped). This
change also refactors the logic slightly so that future additions can
be clearer (no need to have if and else-if combinations).
Only lowercase and even-length tag values are stored as binary BLOBs.
Previously there was an error which search results from being returned
if the tag value was mixed-case and could be interpreted as hex.
A new database migration has been created to repair the `tag` table
for existing relays.
fixes: https://todo.sr.ht/~gheartsfield/nostr-rs-relay/37
